1. Field of the Invention
The present invention relates to connectors for fiber optic cables and, more particularly, to multi-channel connectors therefor that can be used in downhole applications and withstand high temperature and pressure.
2. Description of the Related Art
Fiber optic cables have been increasingly used for downhole oil and gas explorations. Specifically, the fiber optic cable is lowered into the well to transmit various information and data to the surface.
The fiber optic cable is typically housed in at least one protective tube to shield the fiber optic from the extremely harsh downhole environment. The fiber optic cable can be subjected to downhole ambient pressures of approximately one thousand (1,000) atmospheres and temperatures ranging from 0° C. to 175° C. (zero to one hundred seventy-five degrees Celsius). Additionally, the fiber optic cable is exposed to downhole corrosives such as water, sulfuric acid and others.
Besides harsh temperatures and pressures, there are a number of other requirements that must be taken into consideration for using fiber optic cable in the downhole applications. For example, packaging for fiber optic cable must be extremely compact for downhole use. Moreover, the fiber optic cable must come either in extremely long segments or be connected. For installation and assembly purposes, it is much easier to have smaller segments of fiber optic cable that connect to each other. However, the connectors for the fiber optic cable must ensure integrity of the transmitted data and information as well as withstand the harsh ambient conditions of the downhole environment.
Existing connectors are intended for the less harsh, surface environment and are typically either multi-channel or single channel. Commercially available multi-channel connectors use physical contact type pins or termini. One such termini is MIL-T-29504 which is manufactured by a number of vendors, one of which is Packard-Hughes Interconnect Corporation. However, these termini rely on the termini preload to cause the polished fiber surfaces to physically distort, thereby minimizing the glass-air interface as a strategy to improve the insertion and return loss performance of the termini. Unfortunately, prolonged exposure of the preloaded termini to temperatures in excess of the glass transition temperature of the epoxy used in bonding of the fiber to the termini will cause the fiber to push back, thereby compromising the geometric requirements for both return and insertion loss. Additionally, the connectors that would incorporate these termini do not meet temperature and pressure requirements for downhole use.
Certain single channel connectors are commercially available with angled termini to reduce the return loss of a physical contact connector. These connectors are manufactured with the end surface of the termini polished at an angle such that the Fresnel reflection at the glass-air interface of the termini is reflected at an angle that exceeds the numerical aperture of the fiber. This allows the return loss (reflected energy) of the connector to be reproducibly suppressed by more than one million times or 60 db. However, existing single-channel connectors are rated for temperatures ranging from −40° C. to 85° C., which is substantially inadequate for downhole use. Additionally, pressure rating of the single-channel angled physical contact connectors is not compatible for downhole use. Moreover, the diameter of the angled termini is at least 2.5 mm, which prohibits inclusion into multi-channel connectors that meet the dimensional requirements of the downhole environment.
Therefore, it is necessary to provide a connector for multi-channel use with fiber optic cable such that the connector can withstand the harsh temperatures and pressures of the well environment as well as have appropriate dimensions for downhole use.